Pedestal antiresonant reflecting waveguides for robust coupling to microsphere resonators and for microphotonic circuits

Whispering-gallery-mode excitation in a microsphere by use of an etched cavity on a multimode fiber end

In this Letter, we demonstrate a whispering-gallery-mode (WGM) microsphere resonator. The device is fabricated by fusion splicing a section of no-core fiber with multimode fiber (MMF) and single-mode fiber at its two ends. The open end of the MMF is etched to create an arc-shaped cavity, with a microsphere glued at its bottom. When the arc of the cavity bottom and microsphere surface are tangent, part of the incident light is coupled into the microsphere and travels exactly one circle before being directed back to the MMF core along the tangent line and exciting the WGM. The device has a Q-factor of 1.21×10⁴, and is robust in structure and low in cost.

Whispering gallery mode sensors

We present a comprehensive overview of sensor technology exploiting optical whispering gallery mode (WGM) resonances. After a short introduction we begin by detailing the fundamental principles and theory of WGMs in optical microcavities and the transduction mechanisms frequently employed for sensing purposes. Key recent theoretical contributions to the modeling and analysis of WGM systems are highlighted. Subsequently we review the state of the art of WGM sensors by outlining efforts made to date to improve current detection limits. Proposals in this vein are numerous and range, for example, from plasmonic enhancements and active cavities to hybrid optomechanical sensors, which are already working in the shot noise limited regime. In parallel to furthering WGM sensitivity, efforts to improve the time resolution are beginning to emerge. We therefore summarize the techniques being pursued in this vein. Ultimately WGM sensors aim for real-world applications, such as measurements of force and temperature, or alternatively gas and biosensing. Each such application is thus reviewed in turn, and important achievements are discussed. Finally, we adopt a more forward-looking perspective and discuss the outlook of WGM sensors within both a physical and biological context and consider how they may yet push the detection envelope further.

Integrated in-fiber coupler for microsphere whispering-gallery modes resonator excitation

We present an integrated in-fiber coupler for excitation of whispering-gallery modes of a microsphere resonator. The coupler is simply fabricated by chemical etching away the holey area of a photonic crystal fiber, leaving a freestanding solid core enclosed in a silica housing. Light is coupled into a microsphere through the suspended core with a diameter of 2.1 μm. Since the coupler itself performs as a Fabry-Perot interferometer, asymmetric Fano resonances can be observed in the mixed reflection spectrum. The silica housing of the coupler provides a robust mechanical support to the microsphere resonator. The new Fano resonance coupler shows great potential in biochemical sensing and optical switching applications.

Fiber pigtailed thin wall capillary coupler for excitation of microsphere WGM resonator

In this paper, we demonstrate a fiber pigtailed thin wall capillary coupler for excitation of Whispering Gallery Modes (WGMs) of microsphere resonators. The coupler is made by fusion-splicing an optical fiber with a capillary tube and consequently etching the capillary wall to a thickness of a few microns. Light is coupled through the peripheral contact between inserted microsphere and the etched capillary wall. The coupling efficiency as a function of the wall thickness was studied experimentally. WGM resonance with a Q-factor of 1.14 × 10(4) was observed using a borosilicate glass microsphere with a diameter of 71 μm. The coupler operates in the reflection mode and provides a robust mechanical support to the microsphere resonator. It is expected that the new coupler may find broad applications in sensors, optical filters and lasers.

Planar coupling to high-Q lithium niobate disk resonators

We demonstrate optical coupling to high-Q lithium niobate disks from an integrated lithium niobate waveguide. The waveguides are made by proton exchange in X-cut lithium niobate substrate. The disks with diameter of 4.7 mm and thickness of 1 mm are made from commercial Z-cut lithium niobate wafers by polishing the edges into a spheroidal profile. Both resonance linewidth and cavity ringdown measurements were performed to calculate the Q factor of the resonator, which is in excess of 10(8). Planar coupling represents the most promising technique for practical applications of whispering gallery mode resonators.

Optical microspherical resonators for biomedical sensing

Optical resonators play an ubiquitous role in modern optics. A particular class of optical resonators is constituted by spherical dielectric structures, where optical rays are total internal reflected. Due to minimal reflection losses and to potentially very low material absorption, these guided modes, known as whispering gallery modes, can confer the resonator an exceptionally high quality factor Q, leading to high energy density, narrow resonant-wavelength lines and a lengthy cavity ringdown. These attractive characteristics make these miniaturized optical resonators especially suited as laser cavities and resonant filters, but also as very sensitive sensors. First, a brief analysis is presented of the characteristics of microspherical resonators, of their fabrication methods, and of the light coupling techniques. Then, we attempt to overview some of the recent advances in the development of microspherical biosensors, underlining a number of important applications in the biomedical field.

Integrated Nd-doped borosilicate glass microsphere laser

A neodymium-doped BK7 glass microsphere laser integrated with a planar ion-exchanged waveguide pumped at 0.8 μm has been demonstrated. The pump radiation was launched by evanescent coupling from the waveguide, and the signal radiation was coupled out through the same waveguide, offering the potential for robustly assembled fully integrated active optical circuits. The dependence of the lasing spectra on pump power and wavelength were studied in detail to clarify the whispering-gallery-mode behavior at the pump and lasing wavelengths.

Enhancement of quality factor for TE whispering-gallery modes in microcylinder resonators

The enhancement of quality factor for TE whispering-gallery modes is analyzed for three-dimensional microcylinder resonators based on the destructive interference between vertical leakage modes. In the microcylinder resonator, the TE whispering-gallery modes can couple with vertical propagation modes, which results in vertical radiation loss and low quality factors. However, the vertical loss can be canceled by choosing appropriate thickness of the upper cladding layer or radius of the microcylinder. A mode quality factor increase by three orders of magnitude is predicted by finite-difference time-domain simulation. Furthermore, the condition of vertical leakage cancellation is analyzed.

Whispering gallery mode spectra of channel waveguide coupled microspheres

Evanescent coupling of light from glass channel waveguides into the whispering gallery modes of glass microspheres of radius 15?m and 100?m is studied experimentally at wavelengths near 1550 nm. Fitting the positions, widths and heights of resonances in the experimental spectra to the characteristic equation for microsphere modes and to universal coupled microresonator theory, we establish sphere radius and index, identify mode numbers, and determine losses. The results provide detailed information for the design of optical devices incorporating microsphere resonators in planar lightwave circuits.

Sol-gel-derived spheres for spherical microcavity

Encapsulation of light to small spheres ensures the highest quality factor ( Q factor) to enhance the interaction between light and materials. In this Account, we describe the fabrication of micrometer-sized spherical particles of organic-inorganic hybrid materials to study the potential ability as a spherical cavity laser. The spherical particles prepared by the vibrating orifice technique included those of nondoped and doped with organic dyes and rare-earth-metal ions, and some of them were cladded with low-index-coating hybrid materials. Coating of the spheres was carried out by aiming at practical applications: high refractive index spheres from n D = 1.72 to 2.5 prepared by the technique and glass spheres of n D = 1.93. They were pumped by second harmonic pulses of a Q-switched Nd:YAG laser (532 nm wavelength) and CW Ar (+) laser (514 nm wavelength) to investigate as spherical cavity microlasers. The emission from spheres originated from the photoluminescence of dopants and Raman scattering of matrix materials. Lasing or resonant light emission from these spheres were performed by the direct-laser-light-pumping and the light-coupling techniques using an optical waveguide coupler.

Universal coupling between metal-clad waveguides and optical ring resonators

We demonstrate excitation of whispering gallery modes in optical ring resonators using a gold-clad pedestal planar waveguide structure. The gold-clad structure provides a strong evanescent field for light-coupling into the resonator while enabling low transmission loss throughout much of the visible and near-infrared region. This is advantageous compared to the previously demonstrated anti-resonant reflecting optical waveguide (ARROW) structure, which can only transmit a narrow wavelength band. We show that the height of the pedestal waveguide can be designed to optimize the coupling conditions for the ring resonator. This technology enhances the practicality of optical ring resonators for sensing devices, laser systems, and many other important applications.

Whispering-gallery-bottle microcavities: the three-dimensional etalon

In a tapered optical fiber there exist localized light structures that, in analogy to the magnetic bottles used in plasma fusion, can be called whispering-gallery bottles (WGBs). These essentially three-dimensional structures are formed by the spiral rays that experience total internal reflection at the fiber surface and that also bounce along the fiber axis in response to reflection from the regions of tapering. It is shown that the Wentzel-Kramers-Brillouin quantization rules for the strongly prolate WGBs can be inversed exactly, thus determining the cavity shape from its spectrum. The approximation considered allows one to find the shape of the etalon bottle, which, similar to the one-dimensional Fabry-Perot etalon, contains an unlimited number of equally spaced wave-number eigenvalues. The problem of determining such a non-one-dimensional cavity is not trivial, because such a cavity does not exist among the uniformly filled cavities such as rectangular boxes, cylinders, and spheroids that allow separation of variables. The etalon cavity corresponds to the fiber radius variation p(z) = rho0/cos(deltakz)/, where deltak is the wave-number spacing. The latter result is in excellent agreement with ray-dynamics numerical modeling.

Whispering-gallery-mode laser at 1.56 mum excited by a fiber taper

We report the observation of whispering-gallery-mode lasers based on Er(3+) -doped ZBLAN microspheres coupled with half-tapered and tapered optical fibers. A multimode lasing effect is observed at 1.56mu when the taper diameters are properly adjusted so that their fundamental propagation constants match those of the resonant modes of interest. Overlap between the pump and the laser geometrical modes is optimized by pumping at 1.48mu .

Microtorus: a high-finesse microcavity with whispering-gallery modes

We demonstrate a 165-mu;m oblate spheroidal microcavity with a free spectral range of 383.7 GHz (3.06 nm), a resonance bandwidth of 23 MHz (quality factor Q approximately 10(7)) at 1550 nm, and finesse F>/=10(4) . The highly oblate spheroidal dielectric microcavity combines a very high Q factor, typical of microspheres, with a vastly reduced number of excited whispering-gallery modes (by 2 orders of magnitude). The very large free spectral range in this novel microcavity-a few hundred gigahertz instead of a few gigahertz as in typical microspheres-is desirable for applications in spectral analysis, narrow-linewidth optical and rf oscillators, and cavity QED.

Planar integrated wavelength-drop device based on pedestal antiresonant reflecting waveguides and high-Q silica microspheres

Whispering-gallery modes in silica microspheres can be accessed very efficiently with the recently introduced stripline pedestal antiresonant reflecting optical waveguide (SPARROW) structure. This integrated-optics coupling technique creates novel application opportunities for the high-Q spherical cavities. We report the demonstration of a narrow-band wavelength-drop configuration utilizing SPARROW waveguides and a silica microsphere.